1. Field of the Invention
This invention relates to thermally conductive polyorganosiloxane elastomers. More particularly, this invention relates to curable polyorganosiloxane compositions containing a unique combination of thermally conductive fillers. Cured elastomers prepared using these compositions are particularly desirable for use as coatings for the fuser rolls used in electrostatic copying machines.
2. Description of the Prior Art
The principles of electrostatic copying are well known. Basically this process involves applying a finely divided, heat fusible toner to a sheet of paper that is electrostatically charged in an image-wise pattern which duplicates the document or other article being copied. The toner particles adhere to the electrically charged areas of the paper and are subsequently fused to form a permanent image by contact with a heated roll conventionally referred to as a fuser roll.
U.S. Pat. No. 3,952,696, which issued to Saupe on Apr. 27, 1976, describes an electrically heated fuser roll coated with an layer of silicone rubber containing electrically conductive particles. No specific fillers are disclosed except by reference to a silicone rubber composition referred to as "R770 VC2".
U.S. Pat. No. 4,360,566, which issued to Shimuzu et al. on Nov. 23, 1982, teaches a silicone rubber composition suitable for use as a fuser roll coating that does not require the presence of silicone oil during the copying process. The rubber is cured by a hydrosilation reaction and contains a filler selected from a lengthy list that includes, but is not limited to silica, quartz, diatomoceous earth, ferric oxide, zinc oxide and alumina.
U.S. Pat. No. 4,444,944, which issued to Matsushita on Apr. 24, 1984, discloses thermally conductive silicone rubber compositions containing alumina of a specified degree of oil absorption and a particle size range of from 2 to 10 microns.
A preference for zinc oxide, based on its high thermal conductivity, as the thickening agent for liquid polymethylalkylsiloxanes intended for use as heat transfer media is disclosed in U.S. Pat. No. 3,885,984, which issued to Wright on May 27, 1984. The alkyl group contains from 4 to 16 carbon atoms.
U.S. Pat. No. 4,243,542, which issued to Mine on Jan. 6, 1981, teaches using up to 33 percent by weight of zinc oxide in electrical insulators that also include a silicone rubber or silicone resin and a compound containing silicon bonded hydrogen atoms.
Thermally conductive silicone rubber compositions containing silica and up to 5 percent by weight of zinc oxide or a zinc salt of a fatty acid are disclosed in U.S. Pat. No. 4,431,701, which issued to Hamada et al. on Feb. 14, 1984, and in Japanese examined application No. 30746/84, published on July 28, 1984.
The optional use of zinc oxide in combination with gamma-alumina and ferric oxide to improve the physical properties of a peroxide cured silicone rubber is taught in U.S. Pat. No. 2,671,069, which issued to Savage on Mar. 2, 1954. The maximum concentration of zinc oxide is 5 weight percent.
Thermally conductive silicone rubber compositions containing from 100 to 800 parts by weight per 100 parts of rubber of at least one oxide selected from beryllia, alumina, hydrated alumina, magnesia and zinc oxide are disclosed in Japanese unexamined application No. 32400/72, published on Nov. 15, 1972. No compositions containing zinc oxide are exemplified.
The use of magnesia as a thermally conductive filler for silicone rubber used in electric heaters is taught in U.S. Pat. No. 4,104,509, which issued to Van Bokestal on Aug. 1, 1978, and in Japanese examined application No. 13891/76, which issued on May 4, 1976.
Finally, fuser roll coatings formed from fluoropolymers, including fluorosilicone elastomers, and the resistance of these polymers to degradation in the presence of hot silicone oil are taught in European Patent Application No. 77991, which was published on May 4, 1983. Silicone oil is often used to remove adhering toner particles and/or documents from the fuser roll.
It should be obvious from the foregoing discussion that a variety of metal oxides have been used as thermally conductive materials in silicone rubbers, which are also referred to as polyorganosiloxane elastomers. The physical and/or chemical properties of some of these oxides make them unsuitable for use as fuser roll coatings. For example, we have found that coating compositions containing alumina, one of the most thermally conductive metal oxides, are eroded relatively rapidly by abrasive materials. These compositions are themselves sufficiently abrasive to damage the underlying surface of the fuser roll and adjacent parts of the electrostatic copier in which the fuser roll is installed.
The hardness exhibited by a polyorganosiloxane elastomer is determined in part by the type and concentration of oxide filler. Relatively low hardness values are desirable to avoid premature eroding of the elastomer and surfaces that come into contact with the elastomer.
An objective of this invention is to provide one- and two part filled polyorganosiloxane compositions that cure by a hydrosilation reaction. The properties of the resultant thermally conductive elastomers make them desirable for use as coatings on the fuser rolls of electrostatic copiers.